PROJECT SUMMARY/ABSTRACT Hexavalent chromium [Cr(VI)] is a common and well-recognized environmental carcinogen causing lung and other cancers, however, the mechanism of Cr(VI) carcinogenesis remains elusive. Previous Cr(VI) carcinogenesis studies mostly focus on its genotoxic effects. However, studies showed that Cr(VI) exposure also causes non- genotoxic effects such as epigenetic changes as evidenced by dysregulations in DNA methylation and histone posttranslational modifications. While these observations open new directions for studying Cr(VI) carcinogenesis, it remains to be determined how Cr(VI)-caused epigenetic dysregulations contribute to Cr(VI) carcinogenesis. Moreover, very little is known about the role of non-coding RNAs (ncRNAs) especially the long non-coding RNAs (lncRNAs), another epigenetic mechanism regulating gene expression, in Cr(VI) carcinogenesis. Recent studies show that lncRNAs are emerging key regulators of gene expression and play critical roles in cancer. The goal of this study is to investigate the mechanism of Cr(VI) carcinogenesis focusing on the role of lncRNA dysregulation. Our preliminary studies found: (i) Chronic Cr(VI) exposure increases the expression of the lncRNA ABHD11-AS1, which contributes causally to Cr(VI)-induced cell transformation, cancer stem cell (CSC)-like property and tumorigenesis; (ii) Chronic Cr(VI) exposure down-regulates the expression of miR-182-5p; but knockdown of ABHD11-AS1 increases the level of miR-182-5p. Moreover, overexpressing miR-182-5p in Cr(VI)-transformed cells significantly reduces their transformed phenotypes, phenocopying the effect of ABHD11-AS1 knockdown; (iii) The Rho GTPase Rac1 and Erk1/2 MAPK are highly activated in Cr(VI)-transformed cells; (iv) The expression level of the oncogenic Rac-GEF Tiam1 is significantly up-regulated in Cr(VI)-transformed cells; but overexpressing miR- 182-5p or knockdown of ABHD11-AS1 greatly decrease Tiam1 level; (v) The expression level of PARP-1, a critical DNA repair gene and a key regulator of gene expression, is significantly up-regulated in Cr(VI)-transformed cells; (vi) The level of protein PARylation is drastically increased in Cr(VI)-transformed cells; inhibition of PARP-1 or knockdown of PARP-1 greatly decrease the levels of protein PARylation and ABHD11-AS1 and the transformed phenotype of Cr(VI)-transformed cells. Based on literature review and our novel preliminary data, our central hypothesis is: ?Chronic Cr(VI) exposure-upregulated lncRNA ABHD11-AS1 sponges miR-182-5p and causes its down-regulation, which leads to increased level of the oncogenic Rac-GEF Tiam1 promoting Cr(VI) carcinogenesis?. Three aims are proposed: Aim 1 will determine the mechanism by which chronic Cr(VI) exposure up-regulates ABHD11-AS1. Aim 2 will demonstrate that ABHD11-AS1 sponges miR-182-5p causing its down- regulation and promoting Cr(VI)-exposure-induced CSC-like property, cell transformation and tumorigenesis. And Aim 3 will demonstrate that down-regulation of miR-182-5p increases levels of the oncogenic Rac-GEF Tiam1 promoting Cr(VI)-exposure-induced CSC-like property, cell transformation and tumorigenesis. Tiam1 knockout mice will be used to demonstrate that Tiam1 plays an important role in Cr(VI)-induced lung carcinogenesis.